Concrete building blocks are widely used in the construction of a variety of structures. A major use of such blocks is in the building of temporary or permanent retaining walls. Blocks used in such walls frequently have interlocking upper and lower surfaces so that the blocks automatically interlock when they are positioned one atop another. This automatic interlocking allows the walls to be built without mortar to expedite the building procedure and to permit the walls to be disassembled, if desired, when they are no longer needed. Because of their mortarless construction and lack of a foundation, the walls require stabilizing. One technique for providing stability is to fill aligned vertical openings in the blocks with reinforced concrete. Known concrete building block systems have limited versatility with regard to both handling of the individual blocks and stabilizing of structures made from the blocks.